Candidates like to preach the preeminence of American values on the campaign trail, but it's interests that dominate inside the White House.

Throughout the history of modern U.S. diplomacy, America’s foreign policy has frequently been torn between two competing and often overlapping tensions: protecting U.S. national security interests and upholding America’s values, particularly as they relate to human rights and democracy promotion. Navigating these two occasionally incompatible impulses has been the bane of many a president’s time in office.

But you might never know such a tension existed if you just listened to the way politicians talked about U.S. foreign policy on the campaign trail. More often than not, those seeking America’s highest office are troubadours of human rights and cynical of any decision that might put "interests" ahead of doing the "right" thing.

Just this month, the values vs. interest debate reared its head again in the one place where it consistently has for much of the past 20 years: China. As U.S. officials — from Kurt Campbell, the assistant secretary of state for East Asian and Pacific affairs, and U.S. Ambassador to China Gary Locke, all the way up to Secretary of State Hillary Clinton — worked feverishly to end a diplomatic imbroglio over the case of Chen Guangcheng, a blind Chinese dissident who had holed up in the U.S. Embassy in Beijing, Republicans took pot shots at Barack Obama’s administration for failing to take a stand with the better angels of American nature.

Indeed, when news stories began trickling out that the Obama administration had forced Chen to leave the U.S. Embassy, Republican candidate Mitt Romney offered a rather restrained statement of displeasure, calling it a "dark day for freedom" and "a day of shame for the Obama administration." According to Romney, "We are a place of freedom, here and around the world, and we should stand up and defend freedom wherever it is under attack."

While Romney’s campaign literature talks about the need to balance U.S. interests and values, this latest attack is very much at pace with Romney’s rhetorical broadsides against the president. According to the Republican standard-bearer, Obama just isn’t that interested in defending American values around the globe. On Iran, he did nothing, claims Romney, as the mullahs wiped out the pro-democracy Green Movement. On Syria, Obama was slow to react and stop the bloodletting. The result, says Romney, is that Obama has turned the Arab Spring into an "Arab Winter." Just last week he argued that the right direction on foreign policy "is to communicate our strength, our determination, and to indicate that if people want to be friends with America, that they’re going to have to hold to the principles that we find dear."

Romney, like many a presidential wannabe, talks a tough game on human rights. But don’t believe a word of it. All presidential candidates, whether Democratic or Republican, prioritize human rights when running for president — less so when they actually reach the office.

More often than not, a new president finds himself coming down on the interests side of the equation. Remember candidateBill Clinton attacking George H.W. Bush in 1992 for meeting with the "butchers of Beijing" after the Tiananmen Square massacre? Several months later, once ensconced in the White House, he backed down, granting China most-favored-nation trading status.

Jimmy Carter ran on a platform of restoring human rights to a prominent place in U.S. foreign policy, and as president he took important steps in that direction. At the same time, however, he found himself torn in knots on the need to protect U.S. interests in the Middle East and Latin America, further détente with the Soviet Union, and recognize communist China — all the while maintaining his pledge to make human rights a foreign-policy focus of his administration. By the end of his presidency, he was moving in a more militaristic, anti-communist direction.

President George W. Bush made democracy promotion the centerpiece of his second-term foreign policy, but within a year he had quickly backed away from the cause in Egypt and Palestine, when political realities got in the way.

In 2008, Obama didn’t talk as much about human rights and democracy promotion. Rather, he spoke of reversing the policies of the Bush administration that threatened civil liberties and undermined America’s image in the world. But at the same time that he ended torture and sought, albeit unsuccessfully, to close the Guantánamo Bay detention center, he also signed on to many of the war on terror policies — such as drone strikes in Yemen, Somalia, and Pakistan — for which Democrats had criticized his predecessor.

In reality, Obama’s human rights record over the last three years has been something of a mixed bag. Supporters can certainly point to the U.S.-led intervention in Libya to support anti-Qaddafi rebels and the efforts to push Egyptian strongman Hosni Mubarak out of power in January 2011. On a multilateral level, the administration has been a big backer of reforming and mobilizing the U.N. Human Rights Council and has used the forum to condemn human rights violators like Syria, Libya, and Iran. On the downside, the White House has continued to back key allies in Yemen, Saudi Arabia, Central Asia, and Bahrain out of a rather cold calculation of U.S. interests. On China, Secretary Clinton received criticismfor appearing to play down human rights during her 2009 visit there, suggesting that the issue should take a back seat to other issues of bilateral importance; it’s a stance that for the most part has been dialed back in the years since. In the end, Obama’s record is that of pragmatist — emphasizing human rights in places where the United States could make a difference and de-emphasizing it in places where it cannot or where national security interests are judged to be more important.

In short, this means Obama has pretty much acted like every president ever does when it comes to human rights.

Now to be sure, what is said on the campaign trail in regard to foreign policy often doesn’t survive once a president finds himself in the Oval Office, but rarely is the backtrack as decisive as it is on human rights. As Julian Zelizer, a professor of history and public affairs at Princeton University, said to me, "No one has found a way to campaign for president and also sound like Henry Kissinger."

Indeed, on the campaign trail, the United States is described as virtuous to a fault, exceptional of course, and above all omnipotent. It’s on this latter point — the issue of U.S. power and influence — where the greatest divide between rhetoric and action can be found.

Romney likes to attack the president for failing to speak up when the Iranian government was shooting pro-democracy protesters "in the streets" — and perhaps Obama should have said more. But one shouldn’t confuse rhetoric with the ability to actually achieve results. In reality, there is pretty much nothing Obama or any president could have said to prevent the bloodletting that accompanied the mass protests in Tehran in the summer of 2009 — and short of going to war, there is little that Romney will be able to doas president to turn Iran into a Jeffersonian democracy.

A similar phenomenon exists with China. The United States can stamp its feet all it wants on Chinese human rights abuses, but doing so is unlikely to reshape Beijing’s behavior (if anything, it’ll do the opposite). As China-watcher Zachary Karabell noted the other day, "Casting American responses to the fate of Chinese activists seeking radical changes in their government as a mark of American weakness says more about American delusions of power than about actual weakness."

In the end, presidents, no matter how powerful they may seem, don’t have all that much power to force other leaders and other countries to bend to the U.S. will (not that they’ve ever had that much to begin with, but at least during the Cold War and the specter of Soviet domination they had a tad more leverage). On human rights, that power is even more constrained by the tension between doing the right thing and doing the best thing for U.S. national security. For 20 years, the United States has been complaining about China’s human rights record and the existence of continued political, social, and cultural repression. But that hasn’t stopped the two countries from developing a rather close economic relationship — and one that both sides went to great lengths to preserve during the Chen crisis.

Indeed, the biggest realization that any presidential candidate will find if they happen to win the presidency is that the power they think they’ve accumulated really ain’t that easy to wield — and that tough talk on human rights is usually only just that.